PHYSIOLOGICAL DOMINANCE 



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The embryonic stages of different animals differ 

 widely as regards their capacity for reconstitution. 

 In the sea-urchin and starfish isolated cells or groups 

 ol cells of the developing embryo down to a cer- 

 tain limit may give rise to complete larvae of small 

 size, while in other forms, such as the annelids and 

 mollusks, isolated parts of the embryo show little or 

 no reconstitutional change, but remain alive for a time 

 and continue to differentiate as they do when they 

 remain as parts of an intact embryo. From the failure 

 of the isolated parts to undergo reconstitution the con- 

 clusion has been drawn that they are independent of 

 each other in the intact embryo, and that development 

 -in these organisms is a sort of mosaic made up of inde- 

 pendent parts with some sort of pre-established harmony 

 between them. If this view is correct, there is no rela- 

 tion of dominance and subordination in these stages 

 of development. The failure of isolated parts to undergo 

 reconstitution does not, however, demonstrate the ab- 

 sence of dominance but merely the ineffectiyeness 

 of isolation. The absence or limitation of embryonic 

 reconstitution in certain forms is apparently due, like 

 the increasing limitation of reconstitutional capacity 

 in higher animals, to the higher specialization of the parts 

 of the egg and embryo in these forms. There is good 

 reason to believe that in such eggs the condition in em- 

 bryonic stages is the result of differentiation dependent 

 upon dominance and subordination of parts in the earlier 

 life of the egg, and that specialization has gone beyond 

 the stage where it can be greatly altered by isolation. 

 Development proceeds in isolated parts as far as it has 

 been determined by past relations with other parts or as 



